A critical period for experience-dependent remodeling of adult-born neuron connectivity.

Neurogenesis in the dentate gyrus (DG) of the adult hippocampus is a process regulated by experience. To understand whether experience also modifies the connectivity of new neurons, we systematically investigated changes in their innervation following environmental enrichment (EE). We found that EE exposure between 2-6 weeks following neuron birth, rather than merely increasing the number of new neurons, profoundly affected their pattern of monosynaptic inputs. Both local innervation by interneurons and to even greater degree long-distance innervation by cortical neurons were markedly enhanced. Furthermore, following EE, new neurons received inputs from CA3 and CA1 inhibitory neurons that were rarely observed under control conditions. While EE-induced changes in inhibitory innervation were largely transient, cortical innervation remained increased after returning animals to control conditions. Our findings demonstrate an unprecedented experience-dependent reorganization of connections impinging onto adult-born neurons, which is likely to have important impact on their contribution to hippocampal information processing.

5-HT1A and 5-HT7 receptor crosstalk in the regulation of emotional memory: implications for effects of selective serotonin reuptake inhibitors.

This study utilized pharmacological manipulations to analyze the role of direct and indirect activation of 5-HT(7) receptors (5-HT(7)Rs) in passive avoidance learning by assessing emotional memory in male C57BL/6J mice. Additionally, 5-HT(7)R binding affinity and 5-HT(7)R-mediated protein phosphorylation of downstream signaling targets were determined. Elevation of 5-HT by the selective serotonin reuptake inhibitor (SSRI) fluoxetine had no effect by itself, but facilitated emotional memory performance when combined with the 5-HT(1A)R antagonist NAD-299. This facilitation was blocked by the selective 5-HT(7)R antagonist SB269970, revealing excitatory effects of the SSRI via 5-HT(7)Rs. The enhanced memory retention by NAD-299 was blocked by SB269970, indicating that reduced activation of 5-HT(1A)Rs results in enhanced 5-HT stimulation of 5-HT(7)Rs. The putative 5-HT(7)R agonists LP-44 when administered systemically and AS19 when administered both systemically and into the dorsal hippocampus failed to facilitate memory. This finding is consistent with the low efficacy of LP-44 and AS19 to stimulate protein phosphorylation of 5-HT(7)R-activated signaling cascades. In contrast, increasing doses of the dual 5-HT(1A)R/5-HT(7)R agonist 8-OH-DPAT impaired memory, while co-administration with NAD-299 facilitated of emotional memory in a dose-dependent manner. This facilitation was blocked by SB269970 indicating 5-HT(7)R activation by 8-OH-DPAT. Dorsohippocampal infusion of 8-OH-DPAT impaired passive avoidance retention through hippocampal 5-HT(1A)R activation, while 5-HT(7)Rs appear to facilitate memory processes in a broader cortico-limbic network and not the hippocampus alone.

Bidirectional modulation of classical fear conditioning in mice by 5-HT(1A) receptor ligands with contrasting intrinsic activities.

5-HT(1A) receptors are implicated in the modulation of cognitive processes including encoding of fear learning. However, their exact role has remained unclear due to contrasting contributions of pre- vs. postsynaptic 5-HT(1A) receptors. Therefore, their role in fear conditioning was studied in mice using the selective ligand S15535, which fully activates 5-HT(1A) autoreceptors, yet only weakly stimulates their postsynaptic counterparts. The effects of S15535 were compared to those of the full agonist 8-OH-DPAT and the selective antagonist NAD-299. 8-OH-DPAT dose-dependently (0.01-0.5 mg/kg) and markedly impaired both context- and tone-dependent fear conditioning, as determined by complementary measures of inactivity and freezing. 8-OH-DPAT-mediated impairments were blocked by pre-injection of the selective 5-HT(1A) antagonist WAY100635. S15535 (0.01-5.0 mg/kg) mimicked 8-OH-DPAT in predominantly impairing conditioned contextual fear, though with smaller effect size than 8-OH-DPAT, consistent with lower efficacy at postsynaptic 5-HT(1A) receptors. Furthermore, S15535 (1.0 mg/kg) tended to attenuate the impairment of fear conditioning by 8-OH-DPAT (0.3 mg/kg). In contrast, NAD-299 (0.3 and 1 mg/kg) facilitated contextual freezing. WAY100635 (0.3 mg/kg) prevented the impairment of contextual fear by S15535 (1 and 5 mg/kg), underpinning the role of 5-HT(1A) receptors in the actions of S15535. Collectively, these data indicate that 5-HT(1A) receptor ligands modulate fear conditioning in a bidirectional manner: activation of postsynaptic 5-HT(1A) sites exerts an inhibitory influence, whereas their blockade promote facilitation of fear conditioning. The results with S15535 underscore the importance of ligand efficacy in determining the actions of 5-HT(1A) receptor ligands in fear conditioning and other models of cognitive function.

5-HT(1A) and NMDA receptors interact in the rat medial septum and modulate hippocampal-dependent spatial learning.

Cholinergic and GABAergic neurons in the medial septum/vertical limb of the diagonal band of Broca (MS/vDB) projecting to the hippocampus, constitute the septohippocampal projection, which is important for hippocampal-dependent learning and memory. There is also evidence for an extrinsic as well as an intrinsic glutamatergic network within the MS/vDB. GABAergic and cholinergic septohippocampal neurons express the serotonergic 5-HT(1A) receptor and most likely also glutamatergic NMDA receptors. The aim of the present study was to examine whether septal 5-HT(1A) receptors are important for hippocampal-dependent long-term memory and whether these receptors interact with glutamatergic NMDA receptor transmission in a manner important for hippocampal-dependent spatial memory. Intraseptal infusion of the 5-HT(1A) receptor agonist (R)-8-OH-DPAT (1 or 4 microg/rat) did not affect spatial learning in the water maze task but impaired emotional memory in the passive avoidance task at the higher dose tested (4 microg/rat). While intraseptal administration of (R)-8-OH-DPAT (4 microg) combined with a subthreshold dose of the NMDA receptor antagonist D-AP5 (1 microg) only marginally affected spatial acquisition, it produced a profound impairment in spatial memory. In conclusion, septal 5-HT(1A) receptors appears to play a more prominent role in emotional than in spatial memory. Importantly, septal 5-HT(1A) and NMDA receptors appear to interact in a manner, which is particularly critical for the expression or retrieval of hippocampal-dependent long-term spatial memory. It is proposed that NMDA receptor hypofunction in the septal area may unmask a negative effect of 5-HT(1A) receptor activation on memory, which may be clinically relevant.

Mice heterozygous for both A1 and A(2A) adenosine receptor genes show similarities to mice given long-term caffeine.

Caffeine is believed to exert its stimulant effects by blocking A(2A) and A(1) adenosine receptors (A(2A)R and A(1)R). Although a genetic knockout of A(2A)R eliminates effects of caffeine, the phenotype of the knockout animal does not resemble that of caffeine treatment. In this study we explored the possibility that a mere reduction of the number of A(1)Rs and A(2A)Rs, achieved by deleting one of the two copies of the A(1)R and A(2A)R genes, would mimic some aspects of long-term caffeine ingestion. The A(1)R and A(2A)R double heterozygous (A(1)R-A(2A)R dHz) mice indeed had approximately one-half the number of A(1)R and A(2A)R, and there were little compensatory changes in A(2B) or A(3) adenosine receptor (A(2B)R or A(3)R) expression. The ability of a stable adenosine analog to activate receptors was shifted to the right by caffeine and in A(1)R-A(2A)R dHz tissue. Caffeine (0.3 g/l in drinking water for 7-10 days) and A(1)R-A(2A)R dHz genotype increased locomotor activity (LA) and decreased heart rate without significantly influencing body temperature. The acute stimulatory effect of a single injection of caffeine was reduced in A(1)R-A(2A)R dHz mice and in mice treated long term with oral caffeine. Thus at least some aspects of long-term caffeine use can be mimicked by genetic manipulation of the A(1)R and A(2A)R.

Decreased behavioral activation following caffeine, amphetamine and darkness in A3 adenosine receptor knock-out mice.

We have examined behavioral consequences of genetic deletion of the adenosine A3 receptors in mice. The open field behavior of A3 adenosine receptor knock-out (A3R KO) mice was investigated both under basal conditions and after stimulation with psychostimulants. Adolescent (21 day-old) and adult A3R KO males showed an increase in overall motor activity compared to wild type (WT) males, but the type of activity differed. The motor activity, especially rearing, was also higher in A3R KO compared to WT adult females. A3 receptors have a low affinity for caffeine and it was therefore surprising to find a decreased response to stimulation with either caffeine or amphetamine in A3R KO as compared to WT mice in males as well as females. Telemetry recordings also showed a significantly smaller increase in activity upon darkness in A3R KO. There were no compensatory changes in the mRNA expression of any other adenosine receptor subtypes (A1, A2A and A2B) or any changes in dopamine D1 and D2 receptor binding in A3R KO brains. Challenge with the developmental toxicant methylmercury (1 microM in drinking water) during pregnancy and lactation did not cause any behavioral alterations in adolescent and adult WT female offspring. In contrast, the A3R KO female offspring displayed changes in locomotion indicating an interaction between perinatal methylmercury and adenosine A3 receptors. In conclusion, despite low expression of A3 receptors in wild type mouse brain we observed several behavioral consequences of genetic elimination of the adenosine A3 receptors. The possibility that this is due to a role of A3 receptors in development is discussed.

5-HT7 receptor stimulation by 8-OH-DPAT counteracts the impairing effect of 5-HT(1A) receptor stimulation on contextual learning in mice.

The principal 5-HT(1A) receptor agonist 8-Hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT) impairs several different types of learning. Besides 5-HT(1A) receptors, 8-OH-DPAT stimulates 5-HT(7) receptors, but it is not known whether 5-HT(7) receptors contribute to the impairments. The 5-HT(7) receptor antagonist (2R)-1-[(3-Hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl] pyrrolidine (SB-269970) was combined with 8-OH-DPAT to dissociate 5-HT(1A) from 5-HT(7) receptor-mediated effects, in the passive avoidance task for emotional learning. SB-269970 intensified impairments caused by 8-OH-DPAT. SB-269970 alone had no effect on memory performance, but moderately decreased retention under suboptimal learning conditions. These findings indicate that 5-HT(7) receptor stimulation by 8-OH-DPAT counteracts 5-HT(1A) receptor-mediated impairments in hippocampal-dependent contextual learning.

Blockade of 5-HT 1B receptors facilitates contextual aversive learning in mice by disinhibition of cholinergic and glutamatergic neurotransmission.

Serotonergic (5-HT) neurotransmission plays a role in learning and memory processes, but the physiological role of various receptor subtypes is not well characterised. Among these, 5-HT(1B) receptors are located as autoreceptors on 5-HT axons and heteroreceptors on non-serotonergic terminals. This study examined the role of the 5-HT(1B) receptor in one-trial aversive contextual learning using the passive avoidance (PA) task in NMRI mice. Subcutaneous administration of the 5-HT(1B) receptor agonist anpirtoline (0.1-1.0mg/kg) before PA training impaired retention performance 24h later. Combined administration of anpirtoline with the selective 5-HT(1B) receptor antagonist NAS-181 (0.1-1.0mg/kg) fully blocked the impairments. Administration of NAS-181 alone dose-dependently improved PA retention performance. This facilitatory effect was blocked by subthreshold doses of both the muscarinic antagonist scopolamine (0.03 mg/kg) and the NMDA receptor antagonist MK-801 (0.03 mg/kg). NAS-181 also fully blocked the PA impairments induced by an amnesic dose of scopolamine (0.1mg/kg), when administered prior to, but not after, scopolamine. In addition, NAS-181 attenuated PA impairments induced by MK-801 (0.3mg/kg). These findings indicate that 5-HT(1B) receptors are activated at basal levels of 5-HT transmission. The facilitatory effect of NAS-181 involved alleviation of an inhibitory 5-HT tone mediated via 5-HT(1B) receptors on cholinergic and glutamatergic transmission. This disinhibition is expected to occur in neuronal circuits involved in contextual learning including the hippocampus and interconnected cortico-limbic regions. Blockade of brain 5-HT(1B) heteroreceptors may represent a novel therapeutic strategy for restoration of deficient cholinergic and glutamatergic neurotransmission contributing to memory disorders.

The role of 5-HT(1A) receptors in learning and memory.

The ascending serotonin (5-HT) neurons innervate the cerebral cortex, hippocampus, septum and amygdala, all representing brain regions associated with various domains of cognition. The 5-HT innervation is diffuse and extensively arborized with few synaptic contacts, which indicates that 5-HT can affect a large number of neurons in a paracrine mode. Serotonin signaling is mediated by 14 receptor subtypes with different functional and transductional properties. The 5-HT(1A) subtype is of particular interest, since it is one of the main mediators of the action of 5-HT. Moreover, the 5-HT(1A) receptor regulates the activity of 5-HT neurons via autoreceptors, and it regulates the function of several neurotransmitter systems via postsynaptic receptors (heteroreceptors). This review assesses the pharmacological and genetic evidence that implicates the 5-HT(1A) receptor in learning and memory. The 5-HT(1A) receptors are in the position to influence the activity of glutamatergic, cholinergic and possibly GABAergic neurons in the cerebral cortex, hippocampus and in the septohippocampal projection, thereby affecting declarative and non-declarative memory functions. Moreover, the 5-HT(1A) receptor regulates several transduction mechanisms such as kinases and immediate early genes implicated in memory formation. Based on studies in rodents the stimulation of 5-HT(1A) receptors generally produces learning impairments by interfering with memory-encoding mechanisms. In contrast, antagonists of 5-HT(1A) receptors facilitate certain types of memory by enhancing hippocampal/cortical cholinergic and/or glutamatergic neurotransmission. Some data also support a potential role for the 5-HT(1A) receptor in memory consolidation. Available results also implicate the 5-HT(1A) receptor in the retrieval of aversive or emotional memories, supporting an involvement in reconsolidation. The contribution of 5-HT(1A) receptors in cognitive impairments in various psychiatric disorders is still unclear. However, there is evidence that 5-HT(1A) receptors may play differential roles in normal brain function and in psychopathological states. Taken together, the evidence indicates that the 5-HT(1A) receptor is a target for novel therapeutic advances in several neuropsychiatric disorders characterized by various cognitive deficits.